SF11 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.73848\,\lambda^2}{\lambda^2 - 0.0136069} + \frac{0.311169\,\lambda^2}{\lambda^2 - 0.061596} + \frac{1.17491\,\lambda^2}{\lambda^2 - 121.923}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.12 \times 10^{-5} + 2(1.81 \times 10^{-8})\,\Delta T + 3(-5.03 \times 10^{-11})\,\Delta T^2 + \frac{1.46 \times 10^{-6} + 2(1.58 \times 10^{-9})\,\Delta T}{\lambda - 0.282}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$